Make ChannelQueue.get_msg true async

[CiprianAnton]

ChannelQueue.get_msg is marked as async, however it's a blocking call.

Looking at https://github.com/jupyter/nbclient/blob/main/nbclient/client.py#L806, there is a call to get_msg which may block infinite. Now that code is fine, because the coroutine is being run as a background task and timeout being handled with asyncio.

However, thinking about a little bit, get_msg will block the entire thread, not giving a chance to other tasks to run.

We can easily make this method true async. I also considered asyncio.Queue, but that is not thread safe and we need it for recv_thread.

[codecov-commenter]

Codecov Report

Merging #892 (0d7d21c) into main (dee8c7a) will increase coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #892      +/-   ##
==========================================
+ Coverage   70.28%   70.44%   +0.16%     
==========================================
  Files          65       65              
  Lines        7683     7698      +15     
  Branches     1284     1287       +3     
==========================================
+ Hits         5400     5423      +23     
+ Misses       1896     1887       -9     
- Partials      387      388       +1     

Impacted Files	Coverage Δ
jupyter_server/gateway/managers.py	63.52% <100.00%> (+3.72%)	⬆️
jupyter_server/services/kernels/kernelmanager.py	78.64% <0.00%> (-0.33%)	⬇️

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[kevin-bates]

The changes look good and make sense - thank you @CiprianAnton!

[blink1073]

@meeseeksdev please backport to 1.x